1. Field of the Invention
The present invention relates to a coated transparent substrate that is used in a flying height tester that measures the height of an air bearing between a disk and a recording head of a hard disk drive.
2. Description of the Background
Hard disk drives contain a number of magnetic recording heads which magnetize and sense a rotating magnetic disk. The recording heads are integrated into a slider which has aerodynamic features that create an air bearing between the head and the rotating disk. The air bearing prevents contact and corresponding mechanical wear of the recording head. A slider is further assembled to a flexure arm to create a head gimbal assembly (HGA).
The sliders are typically tested in a flying height tester before being installed into a hard disk drive assembly. Flying height testers measure the height of the air bearing to insure that the slider complies with manufacturing specifications. Flying height testers typically contain a loader which places each slider adjacent to a rotating transparent disk. A beam of light is directed through the glass disk and reflected off of the slider surface. Multiple reflections occur at the glass/air and slider/air interfaces, creating an interference pattern that is detected and analyzed by the tester to determine the flying height of the slider. The current industry standard is the Dynamic Flying Height Tester (DFHT) made by Phase Metrics, Inc. which is now part of KLA-Tencor, Inc. the assignee of the present application. The DFHT utilizes multiple wavelength intensity based interferometry to determine the flying height.
The measurement sensitivity of DFHT depends on the slider flying height which is increasingly becoming lower and lower to increase the coupling between the magnetic head and disk. Theoretically, the DFHT has zero sensitivity at zero flying height if the slider air bearing surface material possesses no imaginary part (k) for the refractive index. Materials possessing vanishing k in the visible region include BK7 glass and Al2O3. Fortunately, typical air bearing surface materials are AlTiC, a granular material made of Al2O3 and TiC. The k value of AlTiC is such that the zero sensitivity flying height is shifted to about negative 10 nm, making low positive flying height measurement possible for DFHT.
Sliders are increasingly designed to operate in or near contact with the disk. The contact may create undesirable wear on the slider or disk during the flying height test. As the flying height gets lower and lower, there is an increasing concern about the tribology of the slider-disk interface such as the disk-surface smoothness. One possible solution is to apply a thin layer of diamond-like carbon (DLC) coating onto the disk. This, however, has the disadvantage of shifting the intensity minimum from a negative flying height to a positive flying height (due to the refractive index of the slider surface). As a result, the measurement sensitivity of the flying height tester is much reduced at low flying height. In addition, sometimes it is desirable to measure the flying height on the Al2O3 cap where the magnetic transducers are located. The current DFHT does not have the sensitivity to measure on Al2O3 at very low flying height. Furthermore, magnetic head manufacturers are changing the air bearing surface materials, some of which have very low k values making measurement at near contact flying height impossible.
There have been several attempts trying to replace DFHT, which is an intensity based interferometer. There was marketed a flying height tester by Zygo Corp. The technology is explained in U.S. Pat. No. 5,557,399 issued to DeGroot. The Zygo machine utilized a polarized coherent light beam that is reflected from the disk and slider at an oblique angle. In theory, the technology should have high sensitivity at zero flying height. But in practice, it faced many fundamental limitations. The most difficult one is the birefringence in the fast-rotating glass disk due to the centrifugal force in the disk. The birefringence rotates the polarization of the returning light beam and makes accurate flying height measurement extremely difficult. In addition, the birefringence effect is more pronounced at an oblique angle away from normal incidence, and is proportional to the square of the angular velocity of the spinning glass disk.
There has also been developed a flying height tester by the assignee the present application, formerly Phase Metrics and currently KLA-Tencor, which is disclosed in U.S. Pat. No. 6,184,992 in the name of Duran et al. In the Duran patent a birefringence element such as a Savart plate is used to split the light reflected from the slider/disk interface into an ordinary beam and an extraordinary beam, thereby creating a double image of the slider/disk interface. The technology is based on phase shifting interferometry and in principle has very good sensitivity at zero flying height. However, in practice, this technology is not robust on the AlTiC grainy surface and suffers from instrument vibration as well as glass birefringence even in the normal incidence case.
For those skilled in the art, it is easy to understand that for an intensity based interferometer such as DFHT, coating on the glass disk with materials having refractive index smaller than that of the BK7 glass will shift the intensity minimum position more negative. MgF2 is an example of such materials. However, these materials do not satisfy the stringent tribological requirements for flying a slider. Coating materials satisfying the stringent tribological requirements usually have refractive index higher than that of BK7 glass. A thin layer of these coating materials will shift the intensity minimum position in the positive direction, therefore reducing the measurement sensitivity at very low flying height.
Over the years, the DFHT technology has been greatly improved. Examples include 0.01 micro inch gauge capability, automatic image based edge detection, and intelligent calibration algorithms. It would therefore be desirable to provide a transparent disk technology so that the mature DFHT technology can be extended to measuring zero flying height accurately on any slider air bearing surface.
The present invention is a method for providing a disk of an optical tester. The disk comprises a transparent substrate that has a first surface and an opposite second surface. The disk also includes a coating on the first surface of the transparent substrate. An identical coating can be applied to the second surface of the transparent substrate. The coating can have multiple layers of thin films. The coating material can have higher refractive index than BK7 glass. The present invention provides a method to determine the required thickness of the coating to enhance the sensitivity of zero flying height measurement.